The CH attack is a special attack used by a spectrobe. In the first game, While moving (or, to be very risky, staying still), hold the A button to charge an energy meter (the amount of energy needed will vary by spectrobe) and then switch to special mode by pressing the Y button. To activate the attack, press L for the left spectrobe or R for the right one.
In Spectrobes: beyond the portals, each attack will charge the meter. Then press B for CH attack or Y for combination attack.
Note (for spectrobes 1): You can also use obtained geos in special mode by pressing X (three energy bars needed) and thus letting the geo destroy the Krawl. To activate combination attack, charge meter fully and press L+R together.